Pilot injection fuel injection pump

ABSTRACT

A timed pulsed fuel injection device which meters fuel continuously and distributes flow pulses to n nozzles through a rotary distributor pressure cascading device which totally isolates the metering event from variations and dynamic effects in the n injection lines can be built to inject a small pilot pulse prior to the main injection pulse of fuel. Because the small pilot charge can ignite quietly prior to injection of the main charge eliminating the ignition lag of the main charge, pilot injection eliminates diesel noise and knock and permits complete control of combustion phasing in a diesel engine.

[ 1 June 24, 1975 United States Patent 1 1 Showalter 1 1 PILOT INJECTION FUEL INJECTION PUMP [761 Inventor: Merle Robert Showalter, 205 Robins Loop, Waco. Tex. 76705 I221 Filed: May I, 1974 I21] Appl No: 466,036

152] U.S. Cl. 239/533; 123/139 AS; 137/14; 138/30 [51 1 Int. Cl. FOZm 39/00 [58] Field of Search. 123/139 AS, 139 R, 139 AK.

[561 References Cited UNITED STATES PATENTS 3.077.872 2/1963 Allen 123/139 AS 3.187.733 6/1965 Hcintzu. 123/139 AS 3,557,764 1/1971 Knight 123/139 AS 3,752,136 8/1973 Knight .1 123/139 AS 3.810.581 5/1974 Rhine 239/533 TOP LEVEL OF MAIN ROTARY DISTRIBUTOR TOP LEVEL OF PILOT ROTARY DISTRIBUTOR MIDDLE LEVEL OF MAIN ROTARY DISTRIBUTOR MIDDLE LEVEL OF PILOT ROTARY DISTRIBUTOR Primary Era/niner-Manuel A. Antonakas Assistant Examiner-Daniel J. O'Connor Attorney, Agent, or FirmWitherspoon and Lane I 57] ABSTRACT A timed pulsed fuel injection device which meters fuel continuously and distributes flow pulses to n nozzles through a rotary distributor pressure cascading device which totally isolates the metering event from \'ariations and dynamic effects in the n injection lines can be built to inject a small pilot pulse prior to the main injection pulse of fuel. Because the small pilot charge can ignite quietly prior to injection of the main charge eliminating the ignition lag of the main charge, pilot injection eliminates diesel noise and knock and permits complete control of combustion phasing in a diesel engine.

7 Claims, 4 Drawing Figures THIRD (BOTTOM) LEVEL OF MAIN ROTART DISTRIBUTOR THIRD LEVEL OF PILOT /ROTARY DISTRIBUTOR (THIRD LEVEL OF MAIN A PILOT ROTARY DISTRIBUT NEVER OPEN TO THE S M CHAMBER 3i AT THE SAN I 1 PILOT INJECTION FL'EL INJECTION PUMP SUMMARY OF THE INVENTION For a long time it has been known that diesel knock and the effective ignition lag of diesel combustion can be eliminated by a two phase injection process, where a very small pilot charge in injected early enough in the cycle that it ignites prior to the injection of the main charge. The combustion of the very small pilot charge does not cause combustion noise (since the pilot charge is quite small) and the main injection into the already burning pilot charge results in so short an ignition lag that diesel knock is eliminated and the diesel combustion (so long as the rate of main injection is ad equately controlled) is no more noisy than combustion in conventional spark fired engines. Elimination of the combustion roughness and shock waves characteristic of diesel combustion would reduce the weight required of diesel engine structures to something comparable to that of high compression gasoline engines. In addition, the effective elimination of the ignition lag would permit higher RPM operation from diesels, and this also would reduce the weight penalty of diesel engines. Also. the elimination of the ignition lag removes one more variable from the diesel and makes it possible to better design the diesel engine for NO; control. which is a matter of very detailed flow patterns and temperature trajectories which can be better handled if combustion as a function of crank angle is a stable and known function.

It is an object of this invention to produce a practical pilot injection pump which may be used in conjunction with an injection system.

Although the advantages of pilot injection are well known, pilot injection has never been used on production engines because of the difficulties involved in designing an injection system with satisfactory pilot injection. The volumes required in pilot injection are very small (ideally as little as a fifth the volume of fuel required to idle the engine) and conventional diesel pumps already have significant leakage problems injecting idling fuel quantities. The quantity desired for pilot injection is frequently less than the leakage of the injection pump. In addition. there is the matter ofinjection phasing: there are reasons to want to inject the pilot charge between 50 and before top dead center, which is not a convenient time in terms of pump design. Also, there is a question of spray formation: if the nozzle is not to drip. the pilot injection pulse must be quite sharp (sharper than can be obtained with conventional cam profiles), Ideally, the pulse shape and duration for the pilot injection should be almost invariant with engine speed, so that a good sharp spray cloud can be formed even at low engine speeds. This is vitally important at the low rotating speeds of cranking, where any cam driven pump would cause dripping. and consequent difficult starting. It is the present invention that an injection pump operating on the pressure cascading method of myself and Samuel Rhine (Samuel Rhine and Merle Robert Shovvalter TIMED PULSED FUEL INJECTION APPARATUS AND METHOD. Ser. No. 33 .153. filed March 8. 1973) or some other injection pump can be made to be an effective pilot injection pump by addition of two additional pressure cascading accumulators and additional rotary distributor levels to 2 form a pilot injection circuit in addition to the main in jection circuit.

The cascading accumulator system is adaptable to pilot injection because: l accumulator chambers have zero leakage: pressurizing pump leakage does not effect the metering events, (2) accumulator blowdown is very fast and not a function of crankangle, so the pilot injection pulse can be very sharp there are no constraints dictated by cam profiles, and (3) because the rotary distributor is simply a timing device without any large or fluctuating turning forces it is easy to vary pilot injection timing as desired.

IN THE DRAWINGS FIG. 1 illustrates the accumulator chamber used in the cascading injection system,

FIG. 2 shows the pressure-volume behavior of the accumulators and illustrates the pressure cascading fluid transfer process employed in the present invention,

FIG. 3 shows the flow structure of the present inven tion, and

HO. 4 is a block diagram showing the sequence of transfer steps in a cascading accumulator system including both a main fuel pulse metering system and a pilot injection pulse metering system, where each metering system feeds in sequence into the n nozzle lines with their n accumulators.

DETAILED DESCRlPTlON OF THE lNVENTlON FIG. 1 shows an accumulator chamber adaptable to diesel pressures according to my copending patent application entitled Accumulator Chamber. The accu mulator has the property that its chamber volume is invariant below its accumulating pressure P, and chamber volume increases rapidly when chamber pressure within passage 4 exceeds the set pressure P,,,-. A smooth surfaced cylindrical billet of steel 3 is shaped on each end to adjoin with other hydraulic lines and contains within itselfa thin and low volume fuel flow passage including passages 4 which communicate with the outside surface of billet-stop which is embedded in the inside of silicone rubber tube 61, which grips around billet 3 and is sealed around its ends by end clamps 7. The silicone rubber tube 61 is surrounded in a bath of oil 9 which is at a pressure near P,,,- due to strain of its surrounding elastomer chamber housing 8. Oil bath 9 distributes its pressure uniformly on all the surfaces of silicone rubber tubes 61, so that the total pressure with which tube 61 grips billet-stop 3 can be greater than the pressure of the structural strain on tube 61 by the amount of the oil bath pressure (the weak silicone rubber may function as a component in accumulator systems operating at pressures much above the ultimate strength of the silicone rubber because of this oil bath pressure bearing). 1n the drawing in FIG. 1, there is a volume of fuel in pocket 10 formed by stretching away of tube 6] and steel flap 60 wherein fuel is accumu lated, lf pressure in passage 4 were to fall below the accumulating pressure P,,, the accumulated fuel volume in pocket 10 would be quickly squeezed back into passage 4 and tube 61 and flap 60 would seat firmly on billet stop 3. The accumulator of FIG. 1 is tolerant of fuels for the required period. operates at the very high pressures required in diesel pumps. accumulates or flows out accumulated fuel very fast, and has the required property that it accumulates volume rapidly above a pressure Pm. but has a truly imariant volume below the set accumulating pressure P,,,

P16. 2 plots the pressure-volume behavior of accumulator chambers such as that of FIG. I. and shows how blowdown in a pressure cascading sequence can be accomplished when an accumulator charged at a pressure above its accumulating pressure is brought in contact with an uncharged accumulator with an accumulating pressure substantially below its accumulating pressure. As shown in H0. 2, so long as the accumulated volume ofchamber l (with pressure volume function VtPl.) is less than pressure VMAX all ofthe accu mulatcd volume of chamber 1 will be very quickly transferred to chamber 2 when fluid contact between the two chambers is established by the opening of a fluid rotary distributor, and the fluid volume will be stored at a pressure p lower than the pressure p,, of chamber l. lt is the pressure drop between p,,, and p which powers the fluid transfer between accumulator chambers l and 2.

Similarly, if chamber 2 with its accumulated volume (now cut off from chamber 1) comes in fluid contact with an accumulator chamber 31' (where i designates one of the n accumulator chambers which feeds one of the n injection nozzles) complete fluid transfer of accumulated volume from chamber 2 and chamber 3i will occur so long as accumulated volume of chamber 3i is not enough to drive its pressure up to p The interaction between chambers l and 2 is redundam and forms the metering system of the injection system. Chamber 2, when it is cut off from chamber 1, blows down its stored volume into one of the accumula tors 3i which feeds the nozzle line i to distribute flow pulses to the injection lines at the proper time and in the proper sequence through a rotary distributor. It is the present invention that the n accumulator chambers can be fed at different times by two metering systems, where one metering system feeds the main injection pulses to the accumulator-linenozzle system at one time while a second metering system feeds the flow pulses required for pilot injection at another time. FIG. 3 shows this. FIG. 3 is analogous to FIG. 4 in TlMED PULSED FUEL INJECTION APPARATUS AND METHOD, Ser. No. 339.153. filed Mar. 8. 1973 by Samuel Rhine and Merle Robert Showaltcr. Pressurized fuel from pressure source P is continuously metered through a flow control device comprising variable metering orifice V and diaphram controlled bypass system 24, 5, 6 which maintains the average pressure drop across valve V at the constant valve to assure accurate fuel metering at each setting of valve V. The flow control device has a bypass system 10.1] which short circuits flow above a set maximum pressure downstream of valve V. This bypass assures that metered volume per injection pulse cannot exceed a set value.

Metered fuel flows to accumulator chambers 1 (ch 1) and 2 (ch2): metered fuel is accumulated in chamber I when chamber l is cut off from chamber 2 by rotary distributor level l (L!) and when chamber 1 is connected to chamber 2 through rotary distributor level 1 fuel flows directly into chamber 2. Accumulator chamber 2 is always open to the flow passage of distributor L] and L3 through always open distributor level 2 L2) which is shaped so that the flow path between accumulators l. 2 and 31' does not vary from injection cycle to injection cycle. During the part of each injection cycle when chamber 2 is cut off from chamber 1 and the flow metering dc\ icc. chamber 2 comes in contact (in rotary sequence through distributor L3 from injection cycle to injection cycle) with one ofthe n accumulator chamber 31'. and discharges all its accumulated volume into this chamber 31' to produce the main injection pulse to nozzle i. This process is always completed in the period of contact between chamber 2 and the chamber 32'. The accumulated fuel in chamber 3i is maintained at sufficient pressure to cause nozzle i to discharge at a high rate. The injection rate is reduced from this maximum rate and the fuel pulse is given the desired shape by the setting of needle valve 1' in line 1'. which is controlled along with other matching needle valves to control maximum injection rate as an increasing function of RPM (and perhaps other parameters).

After chamber 2 discharges into chamber 3!. contact between ch2 and ch 31' is cut off by distributor L3 and fluid contact is again re-established through distributor Ll. The cycle then repeats. this time producing injection in the next injection line.

The description of the main injection sequence is now complete. According to the present invention. this injection system can be modified to produce a pilot injection pulse prior to the main injection pulses by the addition of a separate pilot injection metering and timing system feeding the accumulators 3i and the nozzles i at a time when flow from the main injection metering system through distributor L3 is cut offv The pilot system consists of a pilot metering system M feeding an accumulator chamber ch 1' (analogous to the main systems ch 1) in intermittant contact through rotary distributor Ll (analogous to the main systems Distributor Ll) with accumulator ch2 which is in intermittant contact through rotary distributor L3 with the accumulator chambers 31' (passages between L3 and accumulators 31' not shown) when flow between ch 1' and ch 2 is cut off by rotary distributor L1. The rotary distributors L3 and L3 are so arranged that both distributors are not open to the same accumulator chamber 31' at the same time. Accumulators chl and ch 2' are arranged in a pressure cascading sequence with each other and the chambers 31'. in the same manner as the cascading chambers of the main injection metering and timing system.

It is worth noting that the pressure cascading pilot injection system of the present invention maybe attached to a main injection system which is of conventional piston design rather than the pressure cascading arrangement of the main injection system of FIG. 3. For the volumes involved in the pilot injection, accumulators for each injection line are not necessary; so that the pilot injection assembly of FIG. 3 comprising M'Ch1'- Ll 'L2'-Ch2'-L3 may feed lines labeled Ch3l, Ch32. 3. but where said lines Ch3l. 3n do not include accumulators and where said lines Ch3l. 3n are fed their main injection pulses by some conventional injection pump.

Although timing of the injection system with conventional rotary distributors is convenient, other fluid distributors which accomplish the same job might be used. It is. however. convenient to have rotary distributors L1. L2 and 1.3 and also 1.1. L2. L3 each be distribu tor groups rotating on common distributor shafts. If it is tolerable to have the crankangle between the start of the pilot injection and the start of the main injection be invariant. all six distributor levels can be on the same shaft. however. it would be relatively easy to vary the angle between the pilot injection and the main injection if desired. for the rotary distributor is only a timing device (it does not serve. for instance. as a pump driveshaft) and it is. therefore. relatively easy to design a timing structure to vary the pilot injection timing. since the rotary distributor drive and timing system need not withstand any large or fluctuating forces.

The combination of the main and pilot injection system of the instant invention is shown in block diagram form in H0. 4. Note that the metering structure of the main injection metering system and the pilot injection metering system is precisely the same.

Certain aspects of the dynamics of the lines of the present injection system are important to the design of the system. particularly the design of the pilot and main metering systems.

In the injection system, the accumulator blowdown process of the accumulator chambers Si is such that the accumulator output goes from a high flow rate to a Zero flow rate in substantially less than a millisecond. Under these conditions the inertia of the fuel in the injection line maintains enough pressure at the nozzle to continue injection for a period after accumulator blowdown has ceased. During this period the pressure in the line near the accumulator 31 goes very low and may go to zero. Under the conditions where the line pressure near accumulator 3i goes to zero cavities may develop. since liquids have no strength in tension.

The system dynamics which produces the cavitation has the great advantage that the lines are automatically reduced to a low or zero pressure without resort to a separate line depressurizing circuit. Nozzles, therefore, do not drip after the main injection.

However. the presence of cavities at the end of the main injection events results in an interaction between successive main and pilot injection flow events which must be compensated for in the design of the metering systems supplying the pilot and main pulses. or eliminated by resort to a separate circuit to fill cavities left by the main injection to assure that the pilot injection system injects its fuel pulse into a full line.

it is also true that. considering the volume of fuel desired in pilot injection. the bulk compressibility of the fuel in the lines cannot be ignored. and results in a certain "minimum delivery pulse equal to the chamber volume times the variation in fuel density between the pressure of nozzle opening and the (lower) pressure of nozzle closing. Unless the system can be designed to utilize fluid wave effects which will be particularly difficult to control in the presence of cavitation, it will be impossible to deliver a fuel pulse for pilot injection less than the minimum delivery so defined.

At the moment of pilot injection (opening ofdistributor L3 to an accumulator 3i and a line i) it will as a rule be true that pressure in line i will be very small or zero. In order for the pilot injection pulse delivered to charn ber 3i to actually result in a pilot injection through nozzle l'. the quantity delivered to accumulator chamber 3i must be at least equal to the volume of the cavity (if any) plus the volume required to compress the fuel in the line i to a density and pressure sufficient to open the nozzle 1'. The quantity required to pressurize the fuel from zero to the opening pressure ofthc nozzle exceeds the minimum delivery" quantity by the difference in fuel dcnsity between nozzle closing pressure and initial line pressure times the chamber volume.

In general the quantity of cavitation volume at the end of a main injection event is a not overly simple function of quantity delivered and RPM, and the volume of this cavity will frequently exceed the desired quantity of pilot injection. Unless means are included to fill this void between the end of the main injection and the next pilot injection (there is a period generally in excess of 600 crank degrees between these events) the quantity delivered by the pilot metering system will have to be a quite complex function of RPM, etc. Pilot metering is far simpler if it is assured that the pilot system always injects into a full line.

This refilling between main injection and the next pilot injection can be done with line repressurizing circuit" connected to the accumulators 3i, for instance, by another passage 50 in distributor L3 connected to line 52, in turn connected to pressure source 54 times to open after the main injection. where the opening is in fluid contact with a passage maintained at a liquid pressure substantially below nozzle opening pressure, but still positive (for example 10 psi) (for instance by a flow which is forced through a bypass valve which opens at 10 psi). If this passage is opened to a chamber 3i at a pressure above 10 psi the circuit serves as a pressure dump. while on the other hand if there is a vacuum and cavity in the chamber 3i and line i the passage will pump fluid into the chamber i and fill its cavity and respressurize it to something near 10 psi. The noise of cavity collapse at the low pressure of 10 psi will be much less than that of cavity collapse at the much higher pressures of injection, which is an advantage also.

If a line reprcssurizing circuit is used, the dynamics of the pilot injection event is greatly simplified, and the quantity of pilot injection delivered from the metering system can be held constant for minimum delivery injection, or simply varied ifit is desired to vary the quantity of pilot injection (it will probably be best to hold pilot injection near the minimum delivery at all times). However, with a line repressurizing circuit, the quantity of fuel delivered to the engine will be the sum of fuel delivery from the main circuit plus fuel delivery from the pilot circuit plus fuel delivered through the line repressurizing circuit. Since the cavity volume following the pilot injection will generally be negligible or zero, practically all of the fuel delivered by the line reprcssurizing circuit will be ejected as part of the main injection pulse.

Because the pilot injection effectively eliminates the ignition lag of the main charge injected, it may perhaps be permissible (with some care in air flow and injection nozzle design) to tolerate a main injection pulse which is not controlled as a function of RPM. Normally. it is desired to control injection so that fuel flow per degree crank angle does not exceed a certain value so pressure rise per degree crank angle does not exceed a predetermined value. but with the effective ignition lag eliminated it may be possible to let mixing limit the peak pressure rise rate per crank degree. since mixing rates will also be proportional to crank angle. in this simple case. the quantity of fuel injected determines the value of velocity at the instant when accumulator blowdown ceases independently of RPM. and the volume of cavitation is therefore only a function of fuel quantity injected during the main injection.

It is not intrinsically necessary for the metering systems of the main and pilot system to be designed as au- 7 tomata. For instance, the pilot system may be controlled (either to vary pilot injection timing or pilot injection quantity) by a servomechanism which sets either pilot advance or pilot injection quantity at the minimum level which eliminates the structural and gas vibrations of diesel knock. since the sound of these \'i brations is easily detected (for instance with a crystal microphone and a bandpass filter to eliminate signals outside of the frequency range of engine vibrations relevant to the servo].

The difficulties caused by fluid compressibility and line cavitation produce some difficulties in the design of the metering system for a pilot injection equipped injection system using a central injection pump and lines of a certain length. These difficulties are not worse in the accumulator cascading injection system than with other systems. and in fact the physics of the interactions in the lines which must be compensated for are a great deal simpler and more stable than the interactions which would have to be compensated for in a conventional piston pump. so that metering system design is a straighforward job with the accumulator system of the present invention.

The cascading accumulator system is adaptable to pilot injection where other pumps are not because: l accumulator chambers have zero leakage: pressurizing pump leakage does not effect the metering events. (2) accumulator blowdown is very fast and not a function of crankangle. so the pilot injection pulse can be very sharp even during startup there are no constraints dictated by cam profiles. and (3) because the rotary distributor is simply a timing device without any large or fluctuating turning forces it is easy to vary pilot injection timing if this is desired. Pilot injection is important because it will make possible great improvements in the performance of diesel engines.

I claim: I. A timed, pulsed fuel injection system including a main injection system and a pilot injection system which injects fuel pulses through it nozzles in a repeating sequence where the main injection system comprises:

a. n 2 accumulator chambers (l). (2), (31). (32).

b. means to meter pressurized fuel flow continuously to accumulator chamber I J;

c. n nozzles (l). (2). (3]. .n where nozzle i is in continuous fluid connection with the corresponding accumulator chamber 31' it l. 2. n);

d. first switching means to open and close fluid flow between the accumulator chambers (I) and (2] and a second switching means to open and close flow between accumulator chamber (2) in succession to accumulator chambers (31 (32). (33). (3n )1 where said first and second switching means open and close fluid contact between the accumulator chambers in the sequence 1-2. 2-31, 1-2. 2-32. [-2. 2-33. 1-2. Z-Bn. l-Z. 2-31. 1-2. and wherein the pilot injection system comprises: pilot accumulator chambers (4) and (5);

l. means to meter pressurized fuel flow continuously to pilot accumulator chamber (4).

. third switching means to open and close fluid flow between the pilot accumulator chambers (4| and t5) and a fourth switching means to open and close flow between pilot accumulator chamber (5) in succession to accumulator chambers (31 (32).

lll

133). (3n) which are held in common by the main and pilot injection systems where said third and fourth switching means open and close fluid contact between the accumulator chambers in the sequence 4-5. 5-3l. 4-5. 5-32. 4-5. 5-33. 4-5. 5-3n. 4-5. 5-3]. 4-5.

h. where each accumulator chamber 1' l {2).(31). (4]. (5.) has a volume versus pressure function r; (p such that (In/zip, t) if pressure p; in the cham ber i is less than a set pressure p,,,-. where dn/dp, is large if p, p,,.. and where the accumulator pressures p,,,- for each chamber are related so that p,, p,, p,, or 2,, or 1,, or or p and so that pu4 pufi pulil or P4131. Ur "1 mm 2. The invention as stated in claim I, and including line repressurizing means to fill any fluid cavities in any of the ii injection lines between discharge of the main injection pulse through said line and the timing of the next pilot injection pulse into said line.

3. The invention as stated in claim 1, and wherein said means to meter pressurized fuel flow continuously to pilot accumulator chamber (4) is adapted to meter a flow of fuel sufficient to fill any cavities in the line i and to pressurize the line i to the opening pressure of the nozzle 1' for each fuel pulse delivered from pilot chamber (5) to said line i.

4. The invention as set forth in claim I and wherein said third and fourth switching means are in the form of a rotary distributor opening and closing fluid contact between the accumulators (4-5) and (3i) (i-l-u] in the sequence 4-5. 5-3l. 4-5. 5-32. 4-5. 5-33 4-5. 5-3n. 4-5. 5-3l. 4-5.

5. A timed. pulsed fuel injection system including a main injection system and a pilot injection system which injects fuel pulses through :2 lines Ll. L2 Ln connected to n nozzles in a repeating sequence com prising;

means to inject main injection pulses through each of said u lines. and

wherein the pilot injection system comprises:

a. pilot accumulator chambers (4) and (5);

b. means to meter pressurized fuel flow continuously to pilot accumulator chamber (4);

c. third switching means to open and close fluid flow between the pilot accumulator chambers (4} and (5) and a fourth switching means to open and close flow between pilot accumulator chamber (Si in succession to lines Ll, L2. ...Ln. where said third and fourth switching means open and close fluid contact between the chambers and lines in the sequence 4-5. 5-1.], 4-5. 5-L2. ...4-5. 5-Ln;

d. where accumulator chambers (4) and (5] have a volume versus pressure function V, [P.) such that (ft (ff); 0 if pressure p in chamber i is less than a set pressure p,,;. where tIH/(IP is large if p p and where accumulator pressures p, are related so that I,, P,,;, and where p,, is greater than the opening pressure of any of the n nozzles; and

e. wherein said main injection system and said pilot injection system are timed so that chamber (5) is not in contact with a line Li when the main injection system is injecting fuel into said line Li.

6. The imcntion as stated in claim 5. and including line repressuri/ing means to fill any fluid cavities in any of the :1 injection lines between discharge of the main injection pulse through said line and the timing of the next pilot injection pulse into said line.

means meters a fuel flow sufficient to fill the cavities of a line Li and to pressurize said line Li t0 the opening pressure of the nozzle 1' under cavitation conditions. 

1. A timed, pulsed fuel injection system including a main injection system and a pilot injection system which injects fuel pulses through n nozzles in a repeating sequence where the main injection system comprises: a. n + 2 accumulator chambers (1), (2), (31), (32), (33), (3n); b. means to meter pressurized fuel flow continuously to accumulator chamber (1); c. n nozzles (1), (2), (3), . . . , n where nozzle i is in continuous fluid connection with the corresponding accumulator chamber 3i (i 1, 2, ..., n); d. first switching means to open and close fluid flow between the accumulator chambers (1) and (2) and a second switching means to open and close flow between accumulator chamber (2) in succession to accumulator chambers (31), (32), (33), ..., (3n); where said first and second switching means open and close fluid contact between the accumulator chambers in the sequence 1-2, 2-31, 1-2, 2-32, 1-2, 2-33, ..., 1-2, 2-3n, 1-2, 2-31, 1-2, . . .; and wherein the pilot injection system comprises: e. pilot accumulator chambers (4) and (5); f. means to meter pressurized fuel flow continuously to pilot accumulator chamber (4); g. third switching means to open and close fluid flow between the pilot accumulator chambers (4) and (5) and a fourth switching means to open and close flow between pilot accumulator chamber (5) in succession to accumulator chambers (31), (32), (33), . . . , (3n) which are held in common by the main and pilot injection systems where said third and fourth switching means open and close fluid contact between the accumulator chambers in the sequence 4-5, 5-31, 4-5, 5-32, 4-5, 5-33, . . . , 4-5, 5-3n, 4-5, 5-31, 4-5, . . .; h. where each accumulator chamber i (1), (2), (3i), (4), (5,) has a volume versus pressure function vi (pi) such that dvi/dpi 0 if pressure pi in the chamber i is less than a set pressure pai, where dvi/dpi is large if pi>pai, and where the accumulator pressures pai for each chamber are related so that pa1>pa2>pa31 or pa32 or pa33 or ... or pa3n and so that pa4>pa5>pa31 or pa32 or . . . or pa3n.
 2. The invention as stated in claim 1, and including line repressurizing means to fill any fluid cavities in any of the n injection lines between discharge of the main injection pulse through said line and the timing of the next pilot injection pulse into said line.
 3. The invention as stated in claim 1, and wherein said means to meter pressurized fuel flow continuously to pilot accumulator chamber (4) is adapted to meter a flow of fuel sufficient to fill any cavities in the line i and to pressurize the line i to the opening pressure of the nozzle i for each fuel pulse delivered from pilot chamber (5) to said line i.
 4. The invention as set forth in claim 1 and wherein said third and fourth switching means are in the form of a rotary distributor opening and closing fluid contact between the accumulators (4-5) and (3i) (i-1-a) in the sequence 4-5, 5-31, 4-5, 5-32, 4-5, 5-33 . . . , 4-5, 5-3n, 4-5, 5-31, 4-5, . . . .
 5. A timed, pulsed fuel injection system including a main injection system and a pilot injection system which injects fuel pulses through n lines L1, L2 ..., Ln connected to n nozzles in a repeating sequence comprising: means to inject main injection pulses through each of said n lines; and wherein the pilot injection system comprises: a. pilot accumulator chambers (4) and (5); b. means to meter pressurized fuel flow continuously to pilot accumulator chamber (4); c. third switching means to open and close fluid flow between the pilot accumulator chambers (4) and (5) and a fourth switching means to open and close flow between pilot accumulator chamber (5) in succession to lines L1, L2, ...Ln, where said third and fourth switching means open and close fluid contact between the chambers and lines in the sequence 45, 5-L1, 4-5, 5-L2, ...4-5, 5-Ln; d. where accumulator chambers (4) and (5) have a volume versus pressure function Vi (Pi) such that dvi/dpi 0 if pressure pi in chamber i is less than a set pressure pai, where dvi/dpi is large if pi>pai and where accumulator pressures pai are related so that Pa4>Pa5 and where pa5 is greater than the opening pressure of any of the n nozzles; and e. wherein said main injection system and said pilot injection system are timed so that chamber (5) is not in contact with a line Li when the main injection system is injecting fuel into said line Li.
 6. The invention as stated in claim 5, and including line repressurizing means to fill any fluid cavities in any of the n injection lines between discharge of the main injection pulse through said line and the timing of the next pilot injection pulse into said line.
 7. The invention as stated in claim 5, and wherein the means to meter pressurized fuel to chamber (4) is adapted to compensate for the cavitation producing effects in said lines L1. . . Ln, whereby said metering means meters a fuel flow sufficient to fill the cavities of a line Li and to pressurize said line Li to the opening pressure of the nozzle i under cavitation conditions. 